Characteristics and mechanism studies
on the removal of nitrogenin sequencing batch reactor of submerged biofilm process

LI Jun1*， LI Yuan1，NIE Meisheng2， WANG Baozhen3

1. College of Civil Engineering & Architecture, Beijing Polytechnic University, Beijing, 100022, PRChina;
2. Housing Industrialization Office，Ministry of Construction, Beijing, 100044, PRChina;
3. College of Municipal & Environmental Engineering, Harbin University of Technology , Harbin, 150008,PRChina

ABSTRACT

The article discusses the nitrogen removal characteristics in sequencing batch reactor of submerged biofilm process. It also sets up the mechanism of over storage-nitrogen removal with SND. Nitrogen removal in the anaerobic phase depends largely on the over storage function of the biofilm to the carbonaceous and nitrogenous organism, and nitrogen removal in the aerobic phase depends on the SND function of biofilm. The organic carbon over storaged in the anaerobic phase in the biofilm provides the major of organic carbon in the process of denitrification.
Keywords：sequencing batch reactor，biological nitrogen removal，simultaneous nitrification and denitrification，over storage

INTRODUCTION

Biofilm process exhibits lots of advantages, such as much higher biomass content in biofilms per volume , much higher ability of resistance to impact, good settling characteristics of the sludge, much more convenient operations and saving energy. At the same time the biofilm attached growing on the carriers can promote microorganism to retain in the reactor and cultivate the dominant bacterium species.
When studying the characteristics of the removal of phosphorus in sequencing batch reactor of submerged biofilm process, the authors in the study found the simultaneous nitrogen removal in the process[1]. It is concerning to further study the characteristics and mechanism of the nitrogen removal exhibited by phosphorus removal in sequencing batch reactor of submerged biofilm process.

TEST METHOD

Test equipment
As shown in Fig.1, the experimental equipment was made of a plexiglass column with an inner diameter of 15cm and volume of 22L, of which the effective volume was 16L, in which the biofilm carriers occupied 6l and the settling zone 2L. Average TP in influent is 10.0mg/L, COD is 370.0mg/L. Temperature is 25℃, and average DO in aerobic status is 5.5mg/L.
Package percentage describes the percentage of biofilm and fibrous carrier in volume to the total volume of the reactor only after the mature of biofilm. Comparative test was made with different package percentage among 37.5%, 30% and 22.5%. Results shows that 30% is the most practical. That means specific surface area of fibrous carrier goes to 2.66m2/L.

1. compressor
2. compressed air regulating vessel
3. rotameter
4. electromagnetic valve
5. diffuser
6. wastewater storage tank
7. outlet of treated wastewater
8. sludge discharge pipe

9. SBR biofilm reactor
10.fibrous carrier package
11.settling zone
12.scmpling point
13.recyling pump
14.automatic control device
15.pressure release valve
16.air distribution pipe

Fig.1 Schematic diagram of experimental equipment

Regarding to culture of biofilm, the author adopted the A/O alternative mode and spent 3 months. Bacterin seed are taken from common active sludge process. The operating procedure of the test is as well as that of the removal of phosphorus in SBR of submerged biofilm process[1] : the period of SBR 9h , of which anaerobic phase of 3h and aerobic phase of 6h . In the following test, sample comes from liquid which after experienced condition changing 2 weeks .

The synthetic wastewater quality and main analysis method
The raw wastewater was prepared by the mixing of a certain amount of peptone, ammonia chloride, magnesium sulfite, calcium chloride and sodium chloride and potassium dihydrogen phosphate. The main quality of parameter of the synthetic wastewater are shown in Table 1.

Table 1 The synthetic wastewater quality

parameter

COD

TN

NH4+-N

NO3--N

NO2--N

TP

SP

PH

Alkalinity

BOD5

Concentration，mg/L

250-400

30-60

10-20

0.2

0.1

8-10

7-9

7.3

380-440

180-300

TEST STUDY

Variation of different nitrogen concentration in aerobic and anaerobic phase

Fig.2 Curves of various nitrogen duringAerobic and anaerobic phase

Fig.3 COD variation curve with differentinfluent COD loading

　

　

We measure the concentration variation curve of the different nitrogen form duration-anaerobic phase 3 hours and aerobic phase of 17 hours in order to study these transformation in aerobic and anaerobic phase. As shown in figure 2, NH4+-N increased, TN decreased dand TN removal efficiency is 34.3% during the anaerobic phase; NH4+-N concentration has already been below 1.0 mg/L after 6 hours of aerobic phase, that means nitrification has been almost finished. At aerobic phase, nitrogen removal efficiency is 22.3% of influent TN and total nitrogen removal efficiency is up to 56.6%. Excessive aerobic phase only transformed the left nitrogen into NO3--N, the total nitrogen removal efficiency did not increased.

The influence on influent COD load

During the test, variation for COD and different forms of nitrogen are observed by applying 4 different COD loading. See figure 3 to figure 7. figure 3 shows that SBR with fibrous carriers can withstand high COD loading. Furthermore, in anaerobic phase, COD absorption rate is high, and high loading brings about high absorbing rate. COD absorption volume is 212.5mg/L at loading of 1.32kg COD/(m3.d) (COD concentration in influent is 496.8mg/l) and 203.1mg/L at loading of 1.00kg COD/(m3.d) (COD concentration in influent is 375.0mg/L). That means when influent loading is 1.00kgCOD/(m3.d), COD absorption has reached its highest value. But it not means all the organic can be used as source of synthetic substance. Therefore, the author chose 0.27~1.32kgCOD/(m3.d) as the proper value for the process.

Fig.4 NO3--N variation curve with different influent COD loading

Fig.5 NO2--N variation curve with different influent COD loading

　

　

Figure 4 concludes 4 kinds of NO3--N variation curve of COD influent load. It indicates the time of nitrification puts off as the increase of the COD influent load. Figure 5 concludes 4 kinds of NO2--N variation curve of COD influent load. It indicates NO2--N reached to a top number and decreased gradually. The time of NO2--N produced and reaching to nitrosation would be prolonged as the increase of the COD influent load. Figure 6 is the corresponding NH4+-N variation curve. It indicates the NH4+-N increased and decreased subsequently as the time passed because the organic nitrogen is transformed into the NH4+-N and produces nitrification. The effluent NH4+-N concentration is 0.4 mg/L at the influent COD load of 1.00 kgCOD/(m3.d), and as is 1.47 mg/L at the influent COD load of 1.32 kgCOD/(m3.d). Figure 7 is the TN removal efficient of 4 kinds of the influent COD load respectively. We can see it has better effects on nitrification and nitrogen removal, at the influent COD load of 0.27~1.32 kgCOD/(m3.d).

Fig.6 NH4+-N variation curve with different influent COD loading

Fig.7 Nitrogen removal with different influent COD loading

　

　

MECHANISM STUDY OF OVER STORAGE NITROGEN REMOVAL IN ANAEROBIC PHASE

The adhesive function of biofilm

The surface of biofilm has highly activated, great holding back and adhesive power. It can adhere to particle, colloidal material and dissolved material in the mix liquid. So there are different micro-organisms, Since autotrophic nitrifiers, heterotrophic bacteria, protozoa and polyose that gather the cells. There are suspended, dissolved and the colloidal organism attaching to the surface of the biofilm floc，microorganism metabolic residuum, the non-degraded group and so on.
Many low molecule dissolved organism can be absorbed directly by the microorganism cell through initiative transport, assistant transport and simple spread mechanism. Though Macromolecule organism of dissolved, suspended and colloidal material are difficult in going across cell wall to cell directly, they can adhere to cell surface, then turn into the dissolving organism that can be transported into cells by the hydrolysis enzyme. Therefore the function of biofilm absorption is very important for organism removal.

Storage and metabolic mechanism

Not all organism attached to the biomembrane can be turned into protoplasm of cell, and a part of them is to exist in the form of ones so-called " store materials". Storage material is dissolving and undissolving organism that are stored in biofilm and the material are utilized by biological metabolism during the microorganism growth.
Carbon organism can be used as glycogen[2], PHB[1] or other chemical compound stored inside the cell. They can also be used as the carbon source and energy that are necessary to the cell growth. These easily degraded storages inside the cell are particularly important in the reasearch, because they are used for nitrify first in the case of anaerobic, and more organism are stored outside the cell, namely, in the biofilm. In a situation that reaction is enough slow outside the cells, after the easily degraded organism stored inside the cells only consume to certain extent, the organism outside the cell can be the carbon source for the denitrification.

Over storage and nitrogen removal mechanism

Submerged SBR biofilm can absorb high concentration of influent organism better in the anaerobic phase. As shown in figure 3. In this phase, microorganism cells have good storage and induced function. The organism including nitrogen and carbon can be accumulated in the cell and these carbon or nitrogen go beyond the needs for the cell growth. James E. Aueman[2] found in his experiment that the glycogen inside the cell increased though the organism is exhausted out of the cell in the aerobic phase. These could prove that the storage organism inside the cell is front body of the glycogen.
To sum up, submerged SBR biofilm reactor takes off nitrogen by 34%. It is because microorganism cell has the good function of absorbing and storing the organism including nitrogen and carbon. These carbon and nitrogen go beyond the needs for growth. More storage material outside the cell (among the biofilm) is utilized by cell through the microorganism metabolism.

SND NITROGEN MECHANISM IN AEROBIC PHASE

According to traditional biological nitrogen removal theory, nitrification and denitrification can not react simultaneously. Nitrification proceeds in the aerobic phase and denitrification proceeds in the anaerobic phase. But in the recently many experiments tested the phenomenon that is simultaneous nitrification and denitrification[3]. Particularly denitrification exists in various biological treatment systems such as Biological Disk[4]、SBR[5]、CAST[6].

Fig.8 Curves of TN and COD during aerobic phase

As we have already said, the organism including nitrogen and carbon over storaged in the anaerobic phase of the SBR biofilm reactor. As shown in figure 8, entering into aerobic phase it also has the duration-nitrogen removal function though the organism outside the cell have already been exhausted(SND function). This tested the storage material in the biofilm has become the organic arbon source in the denitrification process. However, authors think the aerobic biofilm layer and the facultative biofilm layer existed in the biofilm .The denitrification bacteria in the deep layer of the facultative biofilm utilized the storage organism in the biofilm as the carbon source to transform the NOx--N produced among the aerobic biofilm layer into the nitrogen gas. Originally in testing the main function of the nitrogen removal in the aerobic phase is the SND nitrogen removal function in the biofilm. As known by picture 7 , the total nitrogen removal rate increased from 48.3% to 56.6% as the influent COD load from 0.27 kgCOD/(m3.d) to 0.73 kgCOD/(m3.d). it demonstrates the nitrogen removal rate produced by SND increased as the storage inside the biofilm increased. The total nitrogen removal rate decreased from 56.6% to 48.7% as the influent COD load from 0.73 kgCOD/(m3.d) to 1.32 kgCOD/(m3.d). It demonstrates that the excessive COD load made the biofilm thick and thus influenced the nitrification and denitrification process and decreased the total nitrogen removal rate. The figure 2 indicated the organic carbon stored inside biofilm decreased as the period of aerobic phase prolonged, and then the nitrogen removal rate decreased gradually too. The excessive aerobic phase did not improve the efficience of the nitrogen removal because the storage organism in the biofilm has been exhausted after aerobic phase of 6 hours.
So the authors think that SND reaction exists in the biofilm. SND reaction is in the boundary district of the aerobic biofilm layer and the facultative biofilm layer. The carbon source for denitrification is the carbon source over storaged in the anaerobic phase. The nitrogen removal rate produced by SND increased as the rate between carbon and nitrogen increased and the excessive influent COD load made the biofilm thick. Both can influence the effect of SND. During one SBR operation period, the exhaustion of store materials in the biofilm at the end of the aerobic phase is well ready for the excessive storage at the beginning of the next anaerobic period.

CONCLUSION

(1) the phosphorus removal process of the SBR biofilm has better simultaneous nitrogen removal. Under the condition of hydraulic retention time 9 hours ( anaerobic phase of 3 h, aerobic phase of 6 h ), the nitrogen removal can be more than 48.3% while the COD load from 0.27 kgCOD/(m3.d) to 1.32 kgCOD/(m3.d). (2) the nitrogen removal mechanism in the anaerobic phase is the nitrogen removal mechanism of the over storage. The microorganism cells have very fine absorbing and storing organism including nitrogen and carbon function, these carbon and nitrogen go beyond the needs of the growth. More storage material exists outside the cell (in the biofilm) and is utilized by the cell through the biological metabolism. (3) the nitrogen removal mechanism in the aerobic phase is the SND mechanism of the biofilm. SND reaction is in the boundary district of the aerobic biofilm layer and the facultative biofilm layer. The carbon source for denitrification is the carbon source over storaged in the anaerobic phase. The carbon source for denitrification is the carbon source over storaged in the anaerobic phase. The nitrogen removal rate produced by SND increased as the rate between carbon and nitrogen increased and the excessive influent COD load made the biofilm thick. Both can influence the effect of SND.

REFERENCES
[1] Li Jun, Wang Baozhen and Nie Meisheng. A study on the removal of phosphorus in sequencing batch reactor of submerged biofilm process[J]. China Water & Wastewater Engineering(Chinese). 2001, 17(7):1-5
[2] James E. Aueman et al. Storage-induced denitridication using sequencing batch reactor operation[J]. Wat. Res. , 1980, 14:1483 -1488
[3] Hyungseok Yoo, Kyu-Hong Ahn. Nitrogen removal from synthetic wastewater by simultaneous nitrification and denitrification (SND) via nitrite in an intermittently-aerated reactor[J]. Wat Res, 1999, 33(1):145-154
[4] Y. Watanabe, et al. Simultaneous nitrification and denitrification in micro-aerobic biofilms[J]. Wat.Sci.Tech., 1992, 26(3-4):511-522
[5] Elisabeth V Munch, et al. Simultaneous nitrification and denitrification in bench-scale sequencing batch reactor[J]. Wat. Res., 1996, 30(2):277-284
[6] Mervyn C Goronszy, Gunnar Demouiin and Mark Newland. Aerated nitrification in full-scale activated sludge facilities[J]. Wat Res, 1997, 35(10):103-110

*Corresponding author
Tel： （010）65958512（H） 67391648（O）E-mail : lj21c@263.net

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